Metal-coated steel sheet

ABSTRACT

Provided is a metal coated steel sheet having a coating layer including a metal having a level of Gibbs free energy equal to that of Fe or above and an oxide thereof. Accordingly, the quality of a plated steel sheet may be improved by preventing the generation of bare spots through inhibition of the formation of Mn oxide, Si oxide, or Al oxide on the surface thereof, and simultaneously, the complexity of a manufacturing facility or an increase in manufacturing costs may be minimized. Economic benefits are thus realized.

TECHNICAL FIELD

The present invention relates to a metal coated steel sheet, a hot-dipgalvanized steel sheet, and manufacturing methods thereof, and moreparticularly, to a metal coated steel sheet having excellent surfacequalities which includes a metal coating layer including a metal havinga level of Gibbs free energy equal to that of iron (Fe) or above, and anoxide thereof in a base steel sheet, a hot-dip galvanized steel sheet,and manufacturing methods thereof.

BACKGROUND ART

Hot-dip galvanized steel sheets have been widely used in automobiles,building materials, various structures, and household appliances due tohaving excellent corrosion resistance properties, and in particular, thehigh strengthening of steel sheets has been continuously undertaken inline with recent demands for weight reductions in vehicles. However,since the ductility of steel sheets relatively decreases when thestrength thereof is increased, high-strength steels having improvedductility, such as dual phase (DP) steels having manganese (Mn), silicon(Si), or aluminum (Al) added to a base steel sheet, complex phase (CP)steels, and transformation induced plasticity (TRIP) steels, have beenmanufactured.

However, Mn, Si, or Al, added to steel sheets, may react with a trace ofoxygen existed in an annealing furnace to form a single or complex oxideof Si, Mn, or Al, and thus, bare spots may be generated to degradesurface qualities of the plated steel sheet.

As a typical method for addressing the foregoing limitations, JapanesePatent Application Laid-Open Publication No. 2005-200690 discloses atechnique in which a base steel sheet is coated with metal, such asnickel (Ni), after annealing and cooling to cover Mn oxide, Si oxide, orAl oxide formed on a surface thereof during annealing with the metalcoating layer. In general, a continuous hot-dip galvanizing process isintegrally configured in order to maintain a reducing atmosphere from anannealing process to a galvanizing process. However, with respect to theabove technique, the annealing and galvanizing processes must beseparated in order to allow for an annealing process before the metalcoating process, and thus, a manufacturing facility may be complicatedand manufacturing costs may be increased.

As another typical method for addressing the foregoing limitations,there is provided a technique, in which metal coating is performed inadvance, and annealing and plating are subsequently performed. However,in the case that a temperature of 750° C. or above is used duringannealing, the coated metallic materials may be diffused into a basesteel sheet to this dissipate or thin a metal coating layer, and thus,there may be limitations in substantially preventing the surfacediffusion of Mn, Si, or Al.

Therefore, the need for a economical technique, allowing for the surfacequalities of a plated steel sheet to be improved by preventing thegeneration of bare spots through the inhibition of the formation of Mnoxide, Si oxide, or Al oxide on the surface of the steel sheet, hasrapidly increased.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a metal coated steel sheet,a hot-dip galvanized steel sheet, and manufacturing methods thereofwhich may minimize complexity of a manufacturing facility or an increasein manufacturing costs while improving quality of the galvanized steelsheet by preventing the formation of an oxide of Mn, Si, or Al on thesurface thereof.

According to an aspect of the present invention, there is provided ametal coated steel sheet having a coating layer including a metal havinga level of Gibbs free energy equal to that of iron (Fe) or above and anoxide of the metal.

The coating layer may be included in a metal amount ranging from 0.1g/m² to 3 g/m², based on an equivalent amount of the metal and the oxideof the metal.

Also, the oxide of the metal may be included in a metal amount rangingfrom 0.5 wt % to 5 wt % based on an equivalent amount of oxygen.

Furthermore, the steel sheet may include one or more selected from thegroup consisting of silicon (Si), manganese (Mn), and aluminum (Al) inan amount of 0.2 wt % or above, and may further include one or moreselected from the group consisting of titanium (Ti), boron (B), andchromium (Cr) in an amount of 0.01 wt % or above.

The metal may be one or more selected from the group consisting ofnickel (Ni), Fe, cobalt (Co), copper (Cu), tin (Sn), and antimony (Sb).

According to another aspect of the present invention, there is provideda hot-dip galvanized steel sheet characterized in that on a glowdischarge spectrometer (GDS) graph of the hot-dip galvanized steel sheetsequentially including a base steel sheet, a coating layer of a metalhaving a level of Gibbs free energy equal to that of Fe or above, and ahot-dip galvanized layer, a peak of the metal is disposed closer to thehot-dip galvanized layer than a peak of oxygen.

A content of oxygen at the peak of oxygen may be in a range of 0.05 wt %to 1 wt %.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above, and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

The metal may be one or more selected from the group consisting of Ni,Fe, Co, Cu, Sn, and Sb.

According to another aspect of the present invention, there is provideda method of manufacturing a metal coated steel sheet characterized inthat a surface of a base steel sheet is coated with a solution in whicha molar concentration of SO₄ ²⁻ is equal to 0.7 times to 1.2 times amolar concentration of Ni²⁺, a concentration of Ni²⁺ is in a range of 20g/L to 90 g/L, and a concentration of Ni(OH)₂ is 1 g/L or less, based onan equivalent amount of Ni.

A pH level of the solution may be in a range of 4 to 6.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above, and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

According to another aspect of the present invention, there is provideda method of manufacturing a hot-dip galvanized steel sheet including:coating a surface of a base steel sheet with a solution in which a molarconcentration of SO₄ ²⁻ is equal to 0.7 times to 1.2 times a molarconcentration of Ni²⁺, a concentration of Ni²⁺ is in a range of 20 g/Lto 90 g/L, and a concentration of Ni(OH)₂ is 1 g/L or less, based on anequivalent amount of Ni; heating the coated steel sheet; cooling theheated steel sheet; and hot-dip galvanizing the annealed steel sheet.

A pH level of the solution may be in a range of 4 to 6.

Also, the heating may be performed at a temperature ranging from 750° C.to 900° C.

Furthermore, the hot-dip galvanizing may be performed in a plating bathhaving a temperature ranging from 440° C. to 480° C.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above, and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

The method may further include performing an alloying heat treatment onthe hot-dip galvanized steel sheet at a temperature ranging from 480° C.to 600° C., after the hot-dip galvanizing.

According to an aspect of the present invention, quality of a platedsteel sheet may be improved by preventing the generation of bare spotsthrough inhibition of the formation of a Mn oxide, a Si oxide, or an Aloxide on the surface thereof, and simultaneously, complexity of amanufacturing facility or an increase in manufacturing costs may beminimized. Thus, the present invention may be highly economicallyadvantageous.

DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1( a) is a schematic view illustrating a structure of a steel sheetaccording to a manufacturing process of a comparative example not havinga nickel (Ni) coating, FIG. 1( b) is a schematic view illustrating astructure of a steel sheet according to a manufacturing process of acomparative example having Ni coating but not including Ni oxides(including hydroxide), and FIG. 1( c) is a schematic view illustrating astructure of a steel sheet according to a manufacturing process of thepresent invention;

FIG. 2 is a graph illustrating the results of glow dischargespectrometer (GDS) analysis of a hot-dip galvanized steel sheetaccording to an example of the present invention; and

FIG. 3 is a graph illustrating the results of GDS analysis of a metalcoated steel sheet according to an example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a metal coated steel sheet of the present invention will bedescribed in detail.

The present inventors recognized limitations in a typical technique forinhibiting the formation of oxide of Mn, Si, or Al on the surface of asteel sheet by coating and annealing a metal having a level of Gibbsfree energy equal to that of iron (Fe) or above, such as nickel (Ni),and then galvanizing, and found that, in the case in which the metalhaving a level of Gibbs free energy equal to that of Fe or above as wellas an oxide (including hydroxide) thereof is included, surface diffusionof Mn, Si, or Al is inhibited by the metal oxide, thereby leading to theinvention of a metal coated steel sheet characterized by having acoating layer including the metal having a level of Gibbs free energyequal to that of Fe or above and the oxide of the metal.

In the present invention, the phrase “the metal having a level of Gibbsfree energy equal to that of Fe or above” denotes that a change in Gibbsfree energy per one mole of oxygen during an oxidation reaction of ametal equal to that of iron (Fe) or above.

When a principle of inhibiting the formation of the oxide of Mn, Si, orAl on the surface of a steel sheet is described using Ni as an exampleof the metal having a level of Gibbs free energy equal to that of Fe orabove with reference to FIG. 1, it may be understood that, with respectto FIG. 1( a), Ni coating is not performed before galvanizing and alarge amount of Mn oxide, Si oxide, or Al oxide is formed on the surfaceof a steel sheet during annealing to thus significantly generate barespots, and, with respect to FIG. 1( b), Ni coating is performed beforegalvanizing, but there may still remain limitations in inhibiting theformation of the oxide of Mn, Si, or Al on the surface of a Ni coatinglayer. With respect to FIG. 1( c), it is in accordance with an exampleof the present invention in which Ni coating is performed beforegalvanizing and Ni oxides (including hydroxide) is also included in acoating layer, wherein Mn, Si, or Al may be in contact with NiO to formMnO, SiO₂, or Al₂O₃, NiO may be reduced to precipitate as Ni, and thus,Mn, Si, or Al may not form an oxide by being diffused into the surfaceof the metal coating layer but Mn oxide, Si oxide, or Al oxide may bedisposed on a lower portion of the metal coating layer or an upperportion of a base steel sheet.

The metal having a level of Gibbs free energy equal to that of Fe orabove may be one or more selected from the group consisting of Ni, Fe,cobalt (Co), copper (Cu), tin (Sn), and antimony (Sb), and the reasonfor using these metals is that the above substitution reaction may befacilitated because a level of Gibbs free energy required for theoxidation of Mn, Si, or Al is much lower than levels of Gibbs freeenergy required for the oxidation of the above metals.

The metal and an oxide thereof may be included in an amount ranging from0.1 g/m² to 3 g/m² based on an equivalent amount of the metal. In thecase that the equivalent amount is less than 0.1 g/m², uncoated portionsmay occur because an amount of the coated metal is relatively small, andan upper limit thereof may be 3 g/m² in consideration of economicfactors.

Also, the oxide of the metal acts to form oxides, such as MnO, SiO₂, orAl₂O₃, before Mn, Si, or Al diffuses into the surface of the metalcoating layer, and the oxide of the metal may be included in an amountranging from 0.5 wt % to 5 wt % based on an equivalent amount of oxygen.In the case that the equivalent amount of oxygen is less than 0.5 wt %,it may not be sufficient to oxidize Mn, Si, or Al before Mn, Si, or Aldiffuses into the surface thereof, and in the case in which theequivalent amount is greater than 5 wt %, the amount of the oxide otherthan the metal may significantly increase to thus decrease adhesionbetween the metal coating layer and the base steel sheet.

Furthermore, the steel sheet may include one or more selected from thegroup consisting of Si, Mn, and Al in an amount of 0.2 wt % or above andmay further include one or more selected from the group consisting oftitanium (Ti), boron (B), and chromium (Cr) in an amount of 0.01 wt % orabove. Since the present invention is aimed at preventing the surfacediffusion of Si, Mn, or Al included in the base steel sheet and theformation of the oxide of the metal, the effects of the presentinvention may be maximized in the case that Si, Mn, or Al is included inan amount of 0.2 wt % or above in the base steel sheet. Also, since Ti,B, and Cr components may form concentrated products on the surface ofthe steel sheet, effects of the present invention may be suitable formaximizing the effects in the case in which Ti, B, or Cr is included inan amount of 0.01 wt % or above.

Hereinafter, a hot-dip galvanizing steel sheet of the present inventionwill be described in detail.

According to another aspect of the present invention, there is provideda hot-dip galvanized steel sheet characterized in that on a glowdischarge spectrometer (GDS) graph of the hot-dip galvanized steel sheetsequentially including a base steel sheet, a coating layer of a metalhaving a level of Gibbs free energy equal to that of Fe or above, and ahot-dip galvanized layer, a peak of the metal is disposed closer to thehot-dip galvanized layer than a peak of oxygen, and the metal may be oneor more selected from the group consisting of Ni, Fe, Co, Cu, Sn, andSb.

Referring to FIG. 2, since the hot-dip galvanized steel sheet is formedafter the metal coating and annealing, a substitution reaction of Mn,Si, or Al with an oxide of the metal having a level of Gibbs free energyequal to that of Fe or above, such as NiO, occurs before Mn, Si, or Aldiffuses into the surface of a metal coating layer, and thus, the oxidemay be disposed on the lower portion of the metal coating layer or theupper portion of the base steel sheet. Thus, oxygen included in theoxide is disposed relatively closer to the base steel sheet and isdisposed relatively farther from the hot-dip galvanized layer incomparison to Ni on the GDS graph. Therefore, that the peak of the metalis disposed closer to the hot-dip galvanized layer than the peak ofoxygen on the GDS graph may be interpreted as improving platability bypreventing the surface diffusion of Mn, Si, or Al and the formation ofthe oxide of the metal on the surface of the metal coating layer by themetal oxide.

A content of oxygen at the peak of oxygen may be in a range of 0.05 wt %to 1 wt %. The content of oxygen at the peak of oxygen denotes a contentat a peak of oxygen included in MnO, SiO₂, or Al₂O₃ after the reactionand may be deduced from oxygen included in the nickel oxide remaining inthe metal coating layer before the reaction. That is, the content ofoxygen is decreased as the metal oxide contained in the initial metalcoating layer is reduced to metal by reduction annealing. In the casethat the content thereof is about 0.05 wt % or above at a peak position,the formation of Mn oxide, Si oxide, or Al oxide on the surface of themetal coating layer may be inhibited, and in the case in which thecontent thereof increases to exceed about 1 wt %, adhesion between thecoating layer and the base steel sheet may be reduced.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above, and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

Hereinafter, a method of manufacturing a metal coated steel sheet of thepresent invention will be described in detail. The present inventionprovides a method of manufacturing a metal coated steel sheetcharacterized in that a surface of a base steel sheet is coated with asolution in which a molar concentration of SO₄ ²⁻ is equal to 0.7 timesto 1.2 times a molar concentration of Ni²⁺, a concentration of Ni²⁺ isin a range of 20 g/L to 90 g/L, and a concentration of Ni(OH)₂ is 1 g/Lor less, based on an equivalent amount of Ni.

First, a molar concentration ratio between SO₄ ²⁻ ions and Ni²⁺ ionsplays an important role in a reaction of forming hydroxide at aninterface during a coating reaction, in which the molar concentration ofSO₄ ²⁻ may be equal to 0.7 times to 1.2 times the molar concentration ofNi²⁺. Since SO₄ ²⁻ ions and Ni²⁺ ions form a weak complex compound in asolution, adhesion between the SO₄ ²⁻ ions and the Ni²⁺ ions may berelatively strong in the case that an amount of the SO₄ ²⁻ ions isrelatively larger than that of the Ni²⁺ ions, and thus, a reaction offorming nickel hydroxide by the reaction of Ni²⁺ ions with OH⁻ ionsduring an interfacial reaction between the solution and the steel sheetmay be inhibited. Therefore, the molar concentration of SO₄ ²⁻ may becontrolled to be equal to 1.2 times or less the molar concentration ofNi²⁺. Also, in the case in which the amount of the SO₄ ²⁻ ions isrelatively smaller than that of the Ni²⁺ ions, the reaction of formingnickel hydroxide during the interfacial reaction may be relativelypromoted, and thus, the reaction of reducing Ni²⁺ ions to Ni may beinhibited to significantly increase nickel oxide in the metal coatinglayer. Therefore, the molar concentration of SO₄ ²⁻ may be controlled tobe equal to 0.7 times the molar concentration of Ni²⁺ or above.

Furthermore, the concentration of Ni²⁺ ions included in the metalcoating solution may be in a range of 20 g/L to 90 g/L. In the case thatthe concentration of Ni²⁺ ions in the solution is less than 20 g/L, anappropriate amount of Ni in the metal coating layer may not be secureddue to a low coating efficiency, and in the case that the concentrationof Ni²⁺ ions in the solution is greater than 90 g/L, a nickel salt mayprecipitate according to microscopic changes in the temperature of thecoating solution.

Also, the concentration of Ni(OH)₂ may be 1 g/L or less, based on anequivalent amount of Ni. Ni(OH)₂ may not be included in the coatingsolution. However, in the case in which Ni(OH)₂ is included in thesolution, it may be advantageous in securing the metal oxide in themetal coating layer. However, in the case that the concentration ofNi(OH)₂ is greater than 1 g/L based on the equivalent amount of Ni, thecoating solution becomes turbid to increase an amount of a sludgegenerated, and thus, an upper limit thereof may be controlled to be 1g/L based on the equivalent amount of Ni.

In addition, a pH level of the coating solution plays a very importantrole in co-depositing the metal oxide in the coating layer. That is, areduction reaction (reaction generating hydrogen gas) of H⁺ ions as wellas a reduction reaction of Ni²⁺ ions may also occur at an interfacebetween the steel sheet as an anode and the solution during a metalcoating process, wherein an increase in the pH thereof mayinstantaneously occur at the interface by the reduction reaction of H⁺ions to change a portion of Ni²⁺ ions into nickel hydroxide and thus,co-deposition of the nickel hydroxide may occur in the metal coatinglayer. Therefore, in the case that the pH level of the pre-platingsolution is relatively low, the generation of the nickel hydroxide isprevented, and in the case in which the pH thereof is relatively high, arelatively large amount of the nickel oxide may be co-deposited.Therefore, the pH thereof may be limited to be within a range of 4 to 6for the co-deposition of an appropriate amount of the nickel oxide.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

Meanwhile, the present invention provides a method of manufacturing ahot-dip galvanized steel sheet including: coating a surface of a basesteel sheet with a solution in which a molar concentration of SO₄ ²⁻ isequal to 0.7 times to 1.2 times a molar concentration of Ni²⁺, aconcentration of Ni²⁺ is in a range of 20 g/L to 90 g/L, and aconcentration of Ni(OH)₂ is 1 g/L or less, based on an equivalent amountof Ni; heating the coated steel sheet; cooling the heated steel sheet;and hot-dip galvanizing the annealed steel sheet. A pH level of thesolution may be in a range of 4 to 6.

In the case that the coating is performed with the solution, sincenickel and nickel oxide are appropriately included in the metal coatinglayer, oxides may be formed before Mn, Si, or Al diffuses into thesurface of the metal coating layer even if heating (annealing) isperformed thereafter, and thus, generation of bare spots due to theformation of the oxide of Mn, Si, or Al on the surface thereof may beprevented. Therefore, in the case that cooling and galvanizing areperformed thereafter, excellent platability may be secured to thusimprove surface qualities of the galvanized steel sheet.

Furthermore, the heating may be performed at a temperature ranging from750° C. to 900° C. In the case that the temperature during annealing isgreater than 900° C., since diffusion rate of Mn, Si, or Al may increaseand a large amount of Ni oxides may be reduced to Ni, remaining Ni oxidemay be less, and thus, the surface diffusion of Mn, Si, or Al may not beeffectively inhibited. In the case that the temperature is less than750° C., the annealing may not be sufficiently performed and thus,excellent material characteristics may not be secured.

The hot-dip galvanizing may be performed in a plating bath having atemperature ranging from 440° C. to 480° C. In the case that thetemperature of the plating bath is less than 440° C., since viscosity ofthe plating bath may decrease, the driving of rollers in the platingbath may be difficult, and thus, scratches may occur in the steel sheetdue to the occurrence of slippage. In the case that the temperature ofthe plating bath is greater than 480° C., an amount of evaporated zinc(Zn) may increase and thus, a manufacturing facility may be contaminatedor Zn may be adhered to the steel sheet to cause defects.

Also, the steel sheet may include one or more selected from the groupconsisting of Si, Mn, and Al in an amount of 0.2 wt % or above and mayfurther include one or more selected from the group consisting of Ti, B,and Cr in an amount of 0.01 wt % or above.

Performing an alloying heat treatment on the hot-dip galvanized steelsheet at a temperature ranging from 480° C. to 600° C. may be furtherincluded after the hot-dip galvanizing. A content of Fe in the coatinglayer may be sufficiently secured by controlling the alloying heattreatment temperature to be 480° C. or above. The temperature iscontrolled to be 600° C. or less and thus, a powdering phenomenon, inwhich the coating layer is detached during processing due to therelatively high content of Fe, may be appropriately prevented.

Hereinafter, the present invention will be described in detail,according to specific examples. However, the following examples aremerely provided to allow for a clearer understanding of the presentinvention, rather than to limit the scope thereof.

EXAMPLES

There are no limitations in steels in which the effects of the presentinvention are implemented. However, since a main object of the presentinvention is to prevent the formation of the oxide of Mn, Si, or Al onthe surface of a metal coating layer, steel including Mn, Si, or Al inan amount of 0.2 wt % or above may be used for maximizing the effects.In the present experiments, a 1.2 mm thick cold-rolled transformationinduced plasticity (TRIP) steel sheet including 1.0 wt % of Si, 1.6 wt %of Mn, and 0.03 wt % of Al was used as a base steel sheet.

Ni coating was performed on the steel sheet and the compositions of Nicoating solutions are presented in Table 1. Coating weights of Nicoating layers were measured by analyzing contents of Ni throughinductively coupled plasma (ICP) after dissolving the coating layers,and contents of Ni oxides in the coating layers were measured byquantitatively analyzing an oxygen component contained in each Nicoating layer by measuring distribution of each component in a thicknessdirection of each steel sheet from an interface to a base steel sheetwith a glow discharge spectrometer (GDS). The interface between the Nicoating layer and the base steel sheet was determined as a point wherean amount of a coating material and an amount of base steel intersectedeach other in a GDS graph.

Samples after the Ni coating were reduction annealed at annealingtemperatures presented in Table 1 for 60 seconds and then cooled to 400°C. Thereafter, over aging was performed at 400° C. for 120 seconds andthe samples were then heated to 480° C. Then, the samples were plated bydipping in a galvanizing bath having an effective Al concentration of0.2% for 5 seconds and coating weight was adjusted to 60 g/m² based onone side thereof through air wiping. A temperature of the galvanizingbath was 460° C.

TABLE 1 Ni(OH)₂ Molar (Ni concentration equivalent ratio Annealing Ni²⁺weight, (Ni²⁺ moles/ temperature Category (g/L) g/l) SO₄ ²⁻ moles) pH (°C.) Inventive 30 0 0.8 5.9 800 Example 1 Inventive 40 0.2 0.7 5 800Example 2 Inventive 40 0.5 1 5 800 Example 3 Inventive 50 0.9 1.2 4.2800 Example 4 Inventive 50 0 0.8 4.8 780 Example 5 Inventive 50 0.6 14.4 800 Example 6 Inventive 50 0.1 0.8 5.5 840 Example 7 Inventive 90 00.7 5.8 860 Example 8 Inventive 90 0.5 1.2 5.8 820 Example 9 Inventive90 1 1.2 5.9 820 Example 10 Comparative — — — — 820 Example 1Comparative 30 0 0.5 5.5 800 Example 2 Comparative 50 0.2 1.5 5.3 780Example 3 Comparative 50 0.5 0.8 6.6 800 Example 4 Comparative 70 0.50.9 3.3 800 Example 5 Comparative 50 0.5 0.5 3.1 800 Example 6Comparative 50 0.9 1.7 6.6 800 Example 7

Surfaces of hot-dip galvanized steel sheets having the plating completedwere visually examined and surface qualities were evaluated according tothe presence and degree of bare spots. Oxides contained in theinterfaces between the pre-plated layers and the base steel sheets wereanalyzed by using a transmission electron microscope (TEM) to confirmwhether the oxides were Si oxide, Mn oxide, Al oxide, and/or complexoxides of Si, Mn, and Al. Also, maximum contents of oxygen at theinterfaces between the Ni coating layers and the base steel sheets weremeasured by analyzing components in a depth direction from the surfacesof the coating layers to the base steel sheets with GDS and the resultsthereof are presented in Table 2.

TABLE 2 Ni coating layer Galvanized steel sheet Ni oxides Maximum oxideCoating weight content content (equivalent (equivalent (equivalentweight of Ni, weight of O, weight of O, Surface Category g/m²) wt %) wt%) qualities Inventive 0.1 0.6 0.07 ◯ Example 1 Inventive 0.5 0.8 0.11 ◯Example 2 Inventive 0.8 1.9 0.25 ⊚ Example 3 Inventive 1.0 3.8 0.47 ⊚Example 4 Inventive 1.0 1.9 0.25 ⊚ Example 5 Inventive 1.0 2.7 0.36 ⊚Example 6 Inventive 1.0 1.5 0.22 ⊚ Example 7 Inventive 2.5 0.7 0.14 ⊚Example 8 Inventive 0.5 4.1 0.59 ⊚ Example 9 Inventive 0.5 4.6 0.68 ⊚Example 10 Comparative — — — X Example 1 Comparative 0.3 0.1 0 X Example2 Comparative 0.5 7.6 0.1 Δ Example 3 Comparative 0.5 6.5 0.1 Δ Example4 Comparative 0.5 0.2 0 X Example 5 Comparative 0.5 0.1 0 X Example 6Comparative 1   8.5 0.3 Δ Example 7

Surface qualities: {circle around (∘)} (very good, steel sheet having nobare spot over the entire plated steel sheet), ∘ (good, steel sheet inwhich a small number of point-shaped bare spots having a diameter ofless than 0.5 mm were observed), Δ (poor, steel sheet in which a largenumber of point-shaped bare spots having a diameter ranging from 0.5 mmto 2 mm were observed), and X (very poor, steel sheet in which barespots having a diameter greater than 2 mm were observed).

As illustrated in Tables 1 and 2, with respect to Inventive Examples 1to 10 in accordance with the present invention, Ni coating was performedby using solutions in which a concentration of Ni²⁺ ions in the coatingsolution were in a range of 20 g/L to 90 g/L, a concentration of Ni(OH)₂was in a range of 0 g/L to 1 g/L based on an equivalent amount of Ni, apH thereof was in a range of 4 to 6, and a molar concentration of SO₄ ²⁻was equal to 0.7 times to 1.2 times a molar concentration of Ni²⁺. As aresult, coating weights of Ni corresponded to a range of 0.1 g/m² to 3g/m², amounts of Ni oxides (including hydroxide) corresponded to a rangeof 0.5 wt % to 5 wt %, and contents of oxygen at oxygen peaks satisfieda range of 0.01% to 1%. Therefore, surface qualities of the samples weregood to very good and thus, it may be confirmed that the formation ofoxide of Mn, Si, or Al on the surface of the coating layers was wellprevented.

However, since Ni coating itself was not performed with respect toComparative Example 1, Ni and an Ni oxides were not included. As aresult, the surface diffusion of Mn, Si, or Al was not prevented andthus, surface qualities were very poor.

With respect to Comparative Example 2, since the molar concentration ofSO₄ ²⁻ was equal to 0.5 times the molar concentration of Ni²⁺ which wasless than 0.7 times, the formation of Ni oxides was prevented, and thus,the formation of oxide of Mn, Si, or Al on the surface of the coatinglayer was not effectively prevented. Therefore, a large number of barespots were observed and surface qualities were very poor.

In contrast, with respect to Comparative Example 3, since the molarconcentration ratio was too high, a relatively large amount of Ni oxideswas formed, and thus, adhesion between the coating layer and the basesteel sheet was not good. It may be confirmed that surface qualitieswere poor because the metal coating layer was partially detached by aroll.

Also, with respect to Comparative Example 4, since the pH was relativelyhigh, a relatively large amount of Ni oxides was formed, and thus, poorsurface qualities were obtained as in Comparative Example 3.

With respect to Comparative Example 5, since the pH was relatively low,the formation of Ni oxides was prevented, and it may be confirmed thatvery poor surface qualities were obtained as in Comparative Example 2.

Furthermore, with respect to Comparative Example 6, since both molarconcentration ratio and pH were low, the formation of Ni oxides washighly prevented, and thus, surface qualities were very poor.

Finally, with respect to Comparative Example 7, since both molarconcentration ratio and pH were high, a relatively large amount of Nioxides was formed, and thus, poor surface qualities were obtained.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

The invention claimed is:
 1. A hot-dip galvanized steel sheetcharacterized in that, on a glow discharge spectrometer (GDS) graph ofthe hot-dip galvanized steel sheet comprising, in the following order, abase steel sheet, a coating layer of a metal having a level of Gibbsfree energy equal to that of Fe or above and an oxide of the metal, anda hot-dip galvanized layer, a peak of the metal is disposed closer tothe hot-dip galvanized layer than a peak of oxygen, wherein the metal isone or more selected from the group consisting of Ni, Co, Cu, Sn, andSb, and wherein a content of oxygen at the peak of oxygen is in a rangeof 0.05 wt. % to 1 wt. %.
 2. The hot-dip galvanized steel sheet of claim1, wherein the steel sheet includes one or more selected from the groupconsisting of Si, Mn, and Al in a total amount of 0.2 wt. % or above,and further includes one or more selected from the group consisting ofTi, B, and Cr in a total amount of 0.01 wt. % or above.